(1) Field of the invention
The present invention relates to a barrier layer interposed between a silicon substrate and an aluminum electrode of a semiconductor device. The barrier layer is for preventing mutual diffusion of silicon and aluminum from occurring between the silicon substrate and the aluminum electrode.
In a semiconductor device, such as an integrated circuit (IC) device, contact resistance, which is generated at a metallic contact between a silicon substrate and a metallic electrode, must be as small as possible. As is well known, an insulating layer of silicon oxide layer is formed on the silicon substrate, and then a metallic contact is made between the metallic electrode and the silicon substrate at a contact area provided on the silicon substrate by removing the silicon oxide layer therefrom. Hitherto, aluminum has been commonly used as the metallic electrode because the aluminum can be easily deposited, as an aluminum film, on the silicon substrate so as to have a high adhesive strength with respect to the silicon oxide layer. However, there has been a problem that mutual diffusion of aluminum and silicon has occurred between the silicon substrate and the aluminum electrode (aluminum film). That is, silicon atoms from the silicon substrate and aluminum atoms from the aluminum film mutually diffuse into the aluminum film and the silicon substrate, respectively. Therefore, in a process of fabricating an IC device, the silicon in the silicon substrate tends to diffuse into the aluminum film and the aluminum in the aluminum layer tends to diffuse into the silicon substrate until the amount of the silicon diffused into the aluminum film and that of the aluminum diffused into the silicon substrate has increased up to the solubility limit at the temperature of fabricating the IC device. This mutual diffusion degrades the characteristics of IC device, producing "pits" at the interface between the silicon substrate and the aluminum film. For preventing this mutual diffusion, an aluminum film containing 1 to 2 weight percent (W %) of silicon has been used for the metal electrode, so that the mutual diffusion is stopped up to fabricating temperatures of less than 550.degree. C. However, the fabricating temperature actually exceeds 550.degree. C. during the several steps during fabricating the IC device, whereupon silicon migrates into the aluminum film. This silicon migration into the aluminum film occurs locally in limited regions, called contact holes or via holes, usually produced near the contact area. Since the migrated silicon has high resistivity, the contact area is effectively reduced, causing an increase in the contact resistance. The increased contact resistance can not be ignored in the IC device, particularly where the IC device is a large scale integrated (LSI) circuit device or a very large scale integrated (VLSI) circuit device.
(2) Description of the related art
To solve the above mutual diffusion problem, a barrier layer has been provided between the silicon substrate and the aluminum layer. Generally, the barrier layer is required to have the following characteristics:
(1) the mutual diffusion of the silicon atoms and the aluminum atoms is prevented, by the barrier layer, from passing through the interface between the silicon substrate and the aluminum layer, even at a high processing temperature (this property is called the "barrier property", hereinafter); and
(2) the barrier layer has low resistivity and low contact resistance between the silicon substrate and the barrier layer and between the aluminum layer and the barrier layer.
Refractory metal compounds such as refractory metal nitrides, refractory metal carbides or refractory metal borides are used as the barrier layer. Such barrier layer is usually fabricated by a reactive sputtering technique using a refractory metal as a target and several kinds of gases. In reactive sputtering, the barrier layer is fabricated under various sputtering conditions. The conditions which are suitably varied include: partial pressure of reactive gas, electric power applied to the target, temperature of the silicon substrate, and electrostatic potential at the silicon substrate. Heretofore, there was a tendency for the properties of the barrier layer to be easily influenced by tiny differences in the sputtering conditions. In particular, it was very hard to repeatedly obtain an expected barrier property.
This problem has been ameliorated by annealing the barrier layer in an atmosphere of an oxygen containing gas after sputtering. The main reason for using an oxygen containing gas atmosphere has been because it was believed that, at high temperatures, oxygen produced oxide at grain boundaries in the barrier layer which prevented the mutual diffusion of silicon and aluminum from occurring through the grain boundaries. This is described in "Investigation of TiN Films for Diffusion Barrier in High Temperature Metallization" by Shuichi KANAMORI: in "Shinkuu" No. 29 (September, 1986).
However, there is still another problem in that the properties of the annealed barrier layer have poor reproducibility because of contamination in the barrier layer occurring during the change from the sputtering to the annealing steps. Usually, the sputtering and the annealing are individually performed in a vacuum chamber of a reactive sputtering apparatus and in an annealing furnace, respectively. The sputtering is usually accomplished on only one wafer (which produces a number of IC tips) at a time and each sputtering takes about 1 to 2 minutes. The annealing, however, can be performed on approximately 50 wafers at a time but takes about 50 minutes. Therefore, during the transfer of the wafer from the sputtering step to the annealing step, the wafer is taken out of the vacuum chamber and the barrier layers may be contaminated by the dust and gas in the air, which causes the properties of the barrier layers to change resulting in low reproducibility of the expected property of the barrier layer.
The problem of contamination might be solved if the wafer did not need to be transferred from the vacuum chamber to the annealing furnace. This has been attempted in two ways both of which use only one vacuum chamber for both sputtering and annealing. One of the two ways, which will be called a "first way" hereinafter, was by, after sputtering, only raising the temperature of the silicon substrate, which will be called a "substrate temperature" hereinafter, in the vacuum chamber without adding oxygen; and the other, which will be called a "second way" hereinafter, was to carry out the sputtering in an atmosphere including a very small percentage of oxygen, but without raising the substrate temperature.
The first way is disclosed in a technical paper, called "Ref. (1)" hereinafter, titled "THE PROPERTIES OF REACTIVE SPUTTERED TIN FILMS FOR VLSI METALLIZATION", by the present inventor and others, on p. 205-211 a publication for "VLSI Multilevel Interconnection (V-MIC) Conference" held in June 13-14, 1988. According to Ref. (1), when the barrier layer is formed by reactive sputterinq at high substrate temperature in a vacuum chamber, the density of a formed barrier layer increases as the substrate temperature increases, which is shown in FIG. 1 of this application. That is, as shown in FIG. 1, the density of the titanium nitride (TiN) barrier layer increases up to approximately 4.75 g.multidot.cm.sup.-3 when the substrate temperature is raised to 600.degree. C., which is equal to an increase in the density of TiN of 15% when the substrate temperature is increased from 25.degree. C. to 600.degree. C. This increase in density may be due to enhancing the crystallization of the TiN as the substrate temperature increases. Because of this increase in density, the barrier property of the barrier layer increases. FIG. 1 also shows that the resistivity of a TiN layer decreases, to approximately 35 .mu..OMEGA..multidot.cm, when the substrate temperature is raised up to 600.degree. C., which is about 1/3 of its resistivity decrease at 25.degree. C.
Ref. (1) also shows a relationship between the rate of failure of the barrier layer and the substrate temperature. The failure rate is a newly provided property devised by the inventor for quantitatively evaluating the barrier property. According to Ref. (1), a pattern of a number of via holes, which is usually called a Test Element Group (TEG), is previously formed on a test substrate consisting of a silicon substrate and an aluminum layer formed on the silicon substrate through a barrier layer of TiN. This is for testing whether the barrier layer failures are caused by via holes created by the mutual diffusion of aluminum and silicon, by observing the failure state of the via holes through a microscope and counting the number of failed via holes. Then, the failure rate is defined as the ratio of the number of failured via holes to the total number of via holes. (The failure rate will be also used in the explanation of the present invention hereinafter.) According to Ref. (1), the failure rate decreases as the substrate treatment temperature increases. However, it has been found that it is hard to decrease the failure rate to less than a value of 1% at 600.degree. C. of the substrate temperature. (In the fabrication of IC devices, it is not desirable to raise the substrate temperature to more than 600.degree. C.)
The second way is disclosed in Japanese laid-open application, SHO 59-182208, filed on Mar. 31, 1983 by Takeuchi et al. It has been well known that an oxygen containing gas has been used for increasing the barrier property. However, according to the laid-open application by Takeuchi et al., the presence of oxygen has the disadvantage of increasing the resistivity of the barrier layer. Therefore, the laid-open application discloses that the flow rate of the oxygen gas should be less than 1% in order to keep the resistivity at a value of less than 300 .mu..OMEGA..multidot.cm. However, in the laid-open application, the substrate temperature is room temperature and nothing is shown about increasing the substrate temperature.
As a result, in the art related to barrier layers, it is practically known that: raising the substrate temperature is effective for decreasing the resistivity and the failure rate, however, it has been impossible to decrease the failure rate to less than 1%; and using an oxygen containing gas during the reactive sputtering is effective in decreasing the failure rate, however, the oxygen causes the resistivity of the barrier layer to increase, so that the proportion of the oxygen gas must be limited to less than 1% to keep the resistivity at less than 300 .mu..OMEGA..multidot.cm. Therefore, it has been believed by those skilled in the art that while a barrier layer must be used for avoiding the problems of mutual diffusion, however, there is still a big problem in that it is almost impossible to expect the barrier layer to decrease the failure rate to less than 1% unless the substrate temperature has been raised more than 600.degree. C. or the resistivity is allowed to be more than 300 .mu..OMEGA..multidot.cm. However, in IC devices, particularly in the case of LSI or VLSI device, a substrate temperature of more than 600.degree. C. and/or a resistivity more than 300 .mu..OMEGA..multidot.cm are impermissible. Furthermore, in the case of LSI or VLSI devices, the failure rate is strongly required to be less than 1%.